This invention relates to carbon fiber-reinforced composites, and more particularly to composites composing carbon fiber reinforcement embedded in a carbon, thermoset resin, metal or ceramic matrix. The composites are formed by infiltrating a porous preform comprising carbonized pitch fiber with the matrix component or a precursor thereof, whereby the matrix component is deposited throughout the structure. Carbon-carbon fiber composites exhibiting unusually high thermal conductivity may be formed by depositing carbon within the interstices of the porous preform by in filtration of the preform using known carbon vapor deposition techniques, or by impregnation of the preform with pitch or a cabonizable resin which then is cured and carbonized. Further heat treatment of the carbon-carbon fiber composites may be used to graphitize the carbon components. As used herein, the term "carbon" is intended to include both ungraphitized and graphitized carbon. Thus, carbon fiber preforms and reinforcement may comprise graphitized, partially graphitized or ungraphitized carbon reinforcing fibers or a mixture thereof, and carbon-carbon fiber composites comprising such reinforcement embedded in a matrix of graphitized, partially graphitized or ungraphitized carbon.
The present invention is particularly concerned with carbon-carbon fiber composites intended for use in applications where severe shear stresses will be encountered, for example, by being subjected to circumferential stress. A prime example of such use is a friction disc employed in a disc brake. Such discs are essentially annular in shape, having at least one surface of each disc being provided with a friction-bearing surface. Braking is accomplished through contact between the friction-bearing surfaces of the discs, thereby converting the mechanical energy of the rotating portion of the brake to heat. In addition to withstanding the shearing stresses, the discs thus also are required to act as heat sinks, dissipating high heat loads. Because of its strength, density, heat capacity, thermal conductivity, coefficient of friction and stability to its sublimation temperature (about 3600.degree. C.), carbon has been particularly attractive for use in constructing such disc brakes, particularly where weight is a major consideration such as in aircraft.
In the prior art, composites have generally been fabricated by orienting or directionally aligning the carbon fiber component, which generally has been thought necessary in order to take advantage of fiber strength and enhance mechanical properties of the composite. Fabricating the composite with the desired fiber orientation is more readily accomplished by use of continuous carbon fiber, and such fiber has been preferred over discontinuous fiber for these applications. The primary forms of continuous fiber employed in composite fabrication include woven textile fabric or unidirectional tapes for use in lay-up structures, and continuous fiber yarn or tow, which are used for filament winding and in braided structures. For example, in a commonly-used process for producing carbon composite brake components, annuli are cut from sheets of PAN-based graphite cloth or unidirectional tape, coated with a suitable binder, stacked and then heated suitably to carbonize the binder. However, variations in binder thickness lead to uneven expansion and contraction during the curing and the resulting composite has set up within it internal stresses which may cause cracking and stress failure in use. Alternative processes designed to overcome such problems are also widely used. For example, a layered stack formed from dry fabric annuli may be infiltrated with vapor-deposited carbon to bind the carbon fibers into a rigid structure suitable for impregnation with a carbonizable binder.
Prior art structures formed from stacked fabric or the like necessarily have the reinforcing fiber distributed within and aligned along each of the planes formed by the fabric layer. The interlayer spaces, lacking fiber reinforcement, generally exhibit lower strength than the fabric layers. Some form of reinforcement is thus needed to improve interlayer strength and thereby avoid or reduce failure through delamination.
Needlepunching is widely used in the textile arts to strengthen stacked fabric structures and improve structural integrity. Generally described, needlepunching operations are carded out by forcing barbed needles normally through the stack layers in the thickness direction. A portion of the fiber within the fabric layers is gathered by the barbs and repositioned in the thickness direction, reinforcing the individual fabric layers as well as the stack. The fiber making up the layers is continuous, hence the needlepunching operation necessarily breaks individual filaments when re-orienting them. To avoid or at least minimize such breakage, improved processes wherein staple fiber is included within the structure, either as part of the fabric layer or as alternating layers of staple fiber sheet, have been used to supply staple fiber to the needles for re-orienting in the needlepunching operation. Needlepunching operations have been employed in the art with carbon fiber sheet and tape to provide preform structures having good integrity for use in the production of carbon-carbon fiber reinforced composites.
As noted, uniformity in the carbon-carbon fiber composite structure has been considered important to the integrity and strength of the product, and the art has continually attempted to develop improved methods for providing uniformity in the preform component. Uniform needlepunching, both in terms of evenly-spaced needles and controlled depth of the needlepunching, has been thought to be important to the uniformity of the product. One widely accepted approach to achieving a high degree of control in constructing preforms from layered fabric or tape has been to needle each of the layers to the layer below as it is added. Prior art methods, such as those disclosed in U.S. Pat. No. 4,621,662 and in U.S. Pat. No. 4,955,123, take great care to emphasize the importance of using just such needlepunching procedures, even to the extent of applying the needlepunching to the fabric at the point of contact with the underlying layer as the fabric is wound on a mandrel. More recently, in U.S. Pat. No. 5,217,770, there is disclosed a process of forming a braided, continuous tube from continuous fiber tow or yam which then is flattened into a tape and layered to form an annular structure, each layer being needle punched as it is added.
The needlepunching process has also been applied to layering fabric sheet formed of carbon fiber and coated with carbonizable binders, which may include conductive particulate or fibrous filler. Needlepunching the stack is said to aid penetration of the liquid binder into the interstices of the fabric layers. Distributing the binder and the carbon fiber in the thickness direction by needlepunching provides, after a carbonizing step, reinforcement of the carbon matrix which may improve resistance to delamination.
The needled preform structures are used as substrates for depositing carbon matrix material, thereby providing reinforcement for the matrix carbon in the carbon-carbon fiber reinforced composite. Known vapor deposition techniques may be used to infiltrate and deposit pyrolytic carbon on the fibrous carbon skeleton. Chemical vapor deposition of carbon and impregnation with carbonizable binders have also been used in combination. A substrate formed of layers of fibrous carbon fabric or similar material thus may first be infiltrated with vapor-deposited carbon to bond the fibrous materials, then impregnated with carbonizable filler material, cured and carbonized to provide the dense, fiber-reinforced carbon article. These and other processes are well known and widely disclosed in the art.
As noted, the high degree of fiber alignment within the structure of these prior art composites is intended to take advantage of the strength and dimensional stability of the carbon fiber. However, composites having the entire fiber content aligned in a single direction would necessarily be highly anisotropic in character, exhibiting a high degree of strength and dimensional stability in the fiber direction while suffering greatly reduced strength properties and poor dimensional stability in the transverse direction. To ensure that the strength of the composite, as well as its heat transfer characteristics and other important mechanical properties, will be reasonably uniform and to minimize unidirectional shrinkage which may cause warping and distortion, the fiber direction will be varied throughout the structure, imparting some isotropic character to the composite. When using fabric or the like, the fabricator has had to resort to varying fiber orientation between successive layers of the structure, for example, using radial orientation in one layer, chordal in the next, and so on, thereby providing a composite having characteristics termed quasi-isotropic. As described above, three-dimensional weaving, needlepunching and similar operations are necessarily employed to add through-thickness fiber orientation and improve interlayer strength properties. However, a preform with fully isotropic character in the fiber reinforcement continues to be difficult to attain.
Current methods for producing carbon-carbon fiber reinforced composites exhibit further shortcomings. For most applications, finished carbon parts generally are made to precise dimensions, and their production requires conducting extensive shaping and machining operations on carbonized or fully graphitized carbon-carbon fiber composite blanks. Precision machining operations are expensive to carry out and difficult, and great care is needed with carbon-carbon fiber composites to avoid cracking or other damage. Carbon blanks having substantially the finished shape and dimensions, termed net shape blanks, would reduce the extent of machining needed and significantly lower costs. However, carbonized preforms are generally friable and cannot be readily formed or shaped. Constructing shaped preforms from layered fabric or fiber sheet thus generally requires cutting component parts having the desired final shape from fabric sheet before stacking and needlepunching. Such cutting operations are wasteful and produce considerable quantities of scrap fabric. Even when suitable methods for recycling of the scrap are found, the production and re-processing of scrap further increases the energy and waste disposal burdens already imposed on the manufacturing process, significantly raising the overall cost of producing the carbon article.
Methods for producing carbon-carbon fiber reinforced preform structures from staple or chopped carbon fiber are also disclosed in the art. For example, in U.S. Pat. No. 4,297,307 there is described a process for extruding a thickened or gelled slurry or dispersion of cut carbon fiber in liquid medium such as water to form an elongated ribbon. The liquid medium may include a carbonizable polymeric binder to bind the fiber component on drying. The elongated ribbon is then arranged in a circular pattern to form a flat disc, and dried to remove the water. The orienting effects of the fluid flow in the thickened medium during extrusion permit aligning or orienting the fiber along the flow line, resulting in very low density, non-woven, discontinuous fiber discs with circumferential fiber orientation. The dried disc may then be heated and, if appropriate, carbonized. A plurality of the resulting thin, low-density discs may be stacked to provide the necessary thickness and then subjected to infiltration or impregnation operations as described and carbonized or graphitized to produce carbon discs for use in brakes or the like. As with other layered structures lacking interlayer fiber reinforcement, the resulting layered carbon disc will be subject to delamination failure.
Methods for forming non-woven webs of carbon fiber have also been disclosed in the art, for example in U.S. Pat. No. 4,032,607. According to patentees, particularly attractive webs may be formed from mesophase pitch by melt- or blow-spinning the pitch, air-layering or water-layering the resulting fiber either as-spun or after being chopped, and thermosetting or air-oxidizing the non-woven web to stabilize the structure before carbonizing. Generally, the resulting webs are composed of random filaments rather than filament bundles or tow, and take the form of low density, thin felts and papers with very low bulk densities, generally well below about 0.3 g/cc. Non-woven webs may be suitable for use in forming layered carbon-carbon fiber structures in the same manner as continuous fiber tape and fabric by employing prior art layering and needlepunching operations such as those described herein above. Even after the needlepunching, structures comprising such highly randomized filaments generally will have a low fiber volume and consequently a very low density. Such structures would not provide the strength advantages generally obtained when using dense, high fiber volume structures comprising aligned and oriented continuous fiber, either in woven textile form or as unidirectional fiber tape.
Preform structures fabricated from cut or chopped fiber heretofore available in the art generally are also low in density and lack the mechanical strength necessary for use in carbon-carbon fiber composites. Methods for fabricating suitable carbon-carbon fiber composites with a high fiber volume from discontinuous fiber are unknown in the art, and the carbon composite industry is thus forced to rely primarily on preforms fabricated from oriented and aligned continuous fiber in order to produce carbon-carbon fiber reinforced composites having the strength properties desired for use where high levels of mechanical stress are encountered. Such prior art composites generally also are deficient in heat transfer properties, particularly in the out-of-plane or thickness direction, further limiting their utility.
A method for fabricating thick preforms and carbon-carbon fiber composite blanks having adequate strength properties and good thermal characteristics from cut or chopped fiber, preferably in a net shape and avoiding the use of binders and liquid carriers that add further to the energy and disposal burdens on the manufacturing process, would be particularly valuable to the carbon composites art.